Among various conventional instruments for dental treatment, there are known a handengine, a handpiece or the like which require motive power to be provided by a motor. Such dental treatment instruments as a handengine, a handpiece or the like should be of course controlled to run at a rate suitable for a diseased part or parts of the teeth to be treated. A conventional controller to adjust the running rate of the handpiece or the handengine is designed to be manipulated by a foot of a dentist. More specifically, shown in FIGS. 6 and 7 annexed to this specification are commonly known typical foot controllers, wherein a foot controller designated generally by reference numeral 30 in FIG. 6 is of the construction to be operated by causing a lever 31 to slide horizontally with the dentist's foot thereby to control the variable resistor (not shown) installed within the controller in order to adjust the running rate of the handengine or the handpiece, whilst a foot controller designated generally by reference numeral 40 being of the construction to be operated by stepping on the cover 41 thereby to actuate the variable resistor (not shown) installed within the controller in order to adjust the running rate of the handpiece or the handengine. Either of the lever 31 or the cover 41 of the aforementioned conventional controllers 30, 40 is designed to return to its original position by means of a coiled spring. It will therefore be necessary for a dentist to see that his foot be always in touch with the lever 31 or with the cover 41 in order to control the dental engine or the handengine at a proper running velocity. This will apparently be a tiresome work for a dentist, as he will have to always be careful in keeping his foot in touch with the lever or the cover of the controller during medical treatment. Nervous fatigue will be unavoidable. Physical fatigue will also be unavoidable due to nervous footwork required during medical treatment.